This invention relates generally to a method and apparatus for cooling the heat generated by a gas turbine engine mounted in a compartment and, in particular, to an eductor system that directs gas turbine exhaust gas and surge bleed air into a primary nozzle of the eductor to entrain sufficient cooling airflow to cool the compartment and to cool the gearbox and generator oil.
In addition to their traditional propulsion functions, gas turbine engines are used as auxiliary power units (APUs) aboard many types of aircraft, ground vehicles, and stationary installations to provide continuous shaft and/or pneumatic power. The shaft power is used to drive electric generators, load compressors, hydraulic pumps, or other equipment. The pneumatic power is used by air turbine motors for main engine starting, cabin air-conditioning and pressurization, de-icing, or other components requiring compressed air. When used aboard an aircraft, for example, the APU is typically mounted in a compartment located within the tail cone of the aircraft.
Historically, APUs have only been operated when the aircraft was on the ground. Currently, aircraft need an additional source of power while in flight. To meet this need an APU may be started and operated in flight at high altitudes. During the operation of the APU, heat is rejected into the compartment from numerous sources including the engine skin; exhaust gases, the tailpipe, as well as the engine oil cooler, generator, and other compartment accessories. To prevent the temperature in the compartment from reaching unacceptable levels, a ventilating or cooling airflow must be provided through the compartment.
To remove this heat, an axial, vane type fan driven by the APU gearbox is usually provided to pump cooling air past the oil cooler as well as through the compartment. However, because of their multiplicity of high speed, rotating parts, these fans are susceptible to mechanical failures, which may require that the aircraft be removed from operation. These fans sometimes leak oil into the cooling flow, which may then cover the oil cooler fins resulting in reduced heat transfer and the possibility of an APU automatic shutdown because of excessive oil temperature.
An alternative to fans is a simple exhaust eductor system having a primary nozzle and an exhaust mixing tailpipe. This eductor uses the kinetic energy of the APU exhaust gas to entrain ambient cooling flow through the compartment and over an oil cooler
The APU's shaft power can be delivered to the gearbox and load compressor in one of two engine architectures. In a single shaft direct drive arrangement, the core engine, the load compressor, and the gearbox are all connected to the same shaft and rotate at the same speed. In another arrangement, the core engine compressor and turbine are connected via one shaft while the gearbox and the load compressor are driven by a free turbine via another shaft.
Each of these engine architectures has their advantages and disadvantages. The eductor performance in a free turbine APU can be reduced during no pneumatic condition. The eductor's cooling flow pumping capacity is directly related to the primary flow rate.
In ground servicing of commercial aircraft, where ground crew fuel and provision the aircraft, and the like, certain noise level limits must be maintained to ensure the health and safety of the ground crew. Therefore, the propulsion engines of the aircraft are typically shut down and only an APU remains in use. The APU may be used in ground service to maintain aircraft interior cooling, oil cooling, engine cooling, to generate electricity for interior lighting, and other necessary operations.
FIG. 1 shows a cross-sectional view of a prior art free turbine auxiliary power unit. A core engine turbine 160 may be coaxial with a free turbine 150. The core engine turbine 160 may include a core engine combustor 140 and a core engine shaft 142 that may drive a core engine compressor 162. Inside the core engine turbine may be located a turbine shaft 144 for delivering shaft power from the free turbine 150 to the load compressor 110 and the gearbox 120 driving generator 130. Turbine exhaust may exit through the primary nozzle 30 and the mixing duct 90. The primary nozzle 30 and the mixing duct 90 together function as the eductor. The turbine exhaust exiting the primary nozzle 30 into the mixing duct 90 entrains ambient air through the oil cooler 164. When needed, the free turbine may be burdened by a generator 130, a gearbox 120, and/or a load compressor 110.
In a free turbine engine, the exhaust flow can vary depending on the load demand on the engine. In a free turbine engine at low pneumatic load, but high generator load, the exhaust flow could be considerably lower than when the free turbine engine is at high pneumatic load and high generator load, which would reduce cooling flow pumping while cooling flow demand for the generator and gearbox oil cooling would still be high.
In certain APU operating conditions when the operator shuts off the demand for the pneumatic load (for example after the main engine start completion, the demand for the high pressure air to drive the starter turbine is shut off) the high pressure airflow must be dumped overboard to prevent the load compressor from surging. This air is often dumped in the exhaust tailpipe.
FIG. 2 shows the prior art approach to mixing exhaust flow with surge bleed air and cooling airflow. Exhaust flow 170 from an upstream gas turbine engine (not shown) flows, as a primary driving flow, past turbine 50 and around center body 40 toward mixing plane 100. Cooling air 174 flows from the external environment through oil cooler 60, into cooling flow plenum 80, and downstream through mixing plane 100, entrained by eduction action of the exhaust flow 170. Continuing downstream from the mixing plane 100, surge bleed air 20 flows into mixing duct 90, wherein the surge bleed air 20 may be entrained into the mixture of cooling airflow 174 and exhaust flow 170. During periods of reduced pneumatic load, such as after a main engine start completion when the operator shuts off the demand for the pneumatic load, the primary driving flow from the gas turbine engine (not shown) may be diminished, while the generator load and need for gear box oil cooling may remain high. In this situation, the primary flow may not be sufficient to entrain sufficient cooling airflow 174 within the mixing duct 90 and provide adequate oil cooling. Dumping the surge bleed air 20 downstream of the primary nozzle 30 into the mixing duct 90 can further aggravate the low eductor pumping.
As can be seen, there is a need for an improved apparatus and method for dumping surge bleed air into the primary flow to compensate for diminished pneumatic load and increased need for component cooling while maintaining a correspondingly high cooling flow rate and eductor performance.